228 Non-Isolable Races. 
willingly agree with the prevailing view. In the sphere 
of mutability, on the other hand, matters are entirely 
different. Here species, subspecies, varieties, races, etc. 
arise by mutations which are induced by rendering active 
a hitherto latent or semi-latent character. The first con- 
dition for a desired mutation therefore is the existence 
of the character in question in a latent or semi-latent 
state. Without this nothing can be achieved, at least in 
the present state of science, and it is only in the case of 
semi-latency that we can have any sort of evidence that 
the desired character is present. Horticultural breeders 
are well known to be continually on the lookout for any 
such indication. 1 
But the presence of a latent character is not of itself 
sufficient, according to my experience, to insure the suc- 
cess of an experiment in selection. For many an experi- 
ment has failed in spite of years of labor. 
This proves nothing in itself, because it is often due 
to lack of sufficient experience, and this experience can 
only be acquired by carrying out a successful experiment 
in an analogous case ; in other words, by making exactly 
the same experiment with a related plant, preferably 
with another species of the same genus. 
For this reason I have more than once endeavored 
to breed a race analogous to one already existing in a 
closely related species of the same group, which is either 
on the market, or has appeared in my own cultures. Ex- 
perience has taught me that the end may often be attained 
with greater or less ease according to circumstances ; but 
that in many other cases, so it appears, insurmountable 
obstacles bar the way. 
A very definite and simple case is afforded by the 
1 See Vol. I, Part I, 25, p. 188; and this volume, Part I, 2, p. o. 
